1. Field of the Invention
This invention relates to polishing systems and particularly to chemical mechanical polishing systems and methods using hydrostatic fluid bearings to support a polishing pad.
2. Description of Related Art
Chemical mechanical polishing (CMP) in semiconductor processing removes the highest points from the surface of a wafer to polish the surface. CMP operations are performed on unprocessed and partially processed wafers. A typical unprocessed wafer is crystalline silicon or another semiconductor material that is formed into a nearly circular wafer about one to twelve inches in diameter. A typical processed or partially processed wafer when ready for polishing has a top layer of a dielectric material such as glass, silicon dioxide, or silicon nitride or a conductive layer such as copper or tungsten overlying one or more patterned layers that create projecting topological features on the order of about 1 xcexcm in height on the wafers surface. Polishing smoothes the local features of the surface of the wafer so that ideally the surface is flat or planarized over an area the size of a die formed on the wafer. Currently, polishing is sought that locally planarizes the wafer to a tolerance of about 0.3 xcexcm over the area of a die about 10 mm by 10 mm in size.
A conventional belt polisher includes a belt carrying polishing pads, a wafer carrier head on which a wafer is mounted, and a support assembly that supports the portion of the belt under the wafer. For CMP, the polishing pads are sprayed with a slurry, and a drive system rotates the belt. The carrier head brings the wafer into Contact with the polishing pads so that the polishing pads slide against the surface of the wafer. Chemical action of the slurry and the mechanical action of the polishing pads and particles in the slurry against the surface of the wafer remove material from the surface. U.S. Pat. Nos. 5,593,344 and 5,558,568 describe CMP systems using hydrostatic fluid bearings to support a belt. Such hydrostatic fluid bearings have fluid inlets and outlets for fluid flows forming films that support the belt and polishing pads.
To polish a surface to the tolerance required in semiconductor processing, CMP systems generally attempt to apply a polishing pad to a wafer with a pressure that is uniform across the wafer. A difficulty can arise with hydrostatic fluid bearings because the supporting pressure of the fluid in such bearings tends to be higher near the inlets and lower near the outlets. Also, the pressure profile near an inlet falls off in a manner that may not mesh well with edges of the pressure profile and adjacent inlet so that pressure is not uniform even if the elevated pressure areas surrounding two inlets overlap. Accordingly, such fluid bearings can apply a non-uniform pressure when supporting a belt, and the non-uniform pressure may introduce uneven removal of material during polishing. Methods and structures that provide uniform polishing are sought.
Hydrostatic bearings include or employ one or more of the aspects of the invention to support polishing pads for uniform polishing. In accordance with one aspect of the invention a hydrostatic bearing support in a polishing system provides a fluid flow across fluid pads having compliant surfaces. The support pressure of a fluid film flow from a fluid inlet and across a compliant pad drops more slowly with distance from the fluid inlet than does the support pressure over a rigid pad. Thus, an array of inlets where some or all of the inlets are surrounded by compliant pad can provide a more uniform pressure profile.
In accordance with another aspect of the invention, a fluid flow is varied in a hydrostatic bearing that supports a polishing pad in contact with a wafer or other object being polished. In one case, the fluid flow is periodically reversed by alternately connecting a fluid source to inlets so that fluid flows from the inlets to outlets and then switching the fluid source to the outlets so that fluid flows from the outlets to inlets. Reversing the fluid flow changes the bearing from a configuration in which support pressure is higher over the inlets to a configuration in which support pressure is higher over the outlets. On a time average basis, the support pressure is thus more uniform than if the fluid flow was not reversed. The changes in direction of fluid flow also can introduce vibrations in the polishing pad thereby aiding polishing. Another case of varying the fluid flow introduces pressure variation in the fluid to transmit vibrational energy to the polishing pads. The pressure variation can be introduced, for example, via an electrically controlled valve connected to a fluid source, an acoustic coupling that transfers acoustic energy to the fluid, or a mechanical agitator in the fluid.
In accordance with another aspect of the invention, a hydrostatic bearing includes a large fluid cavity having a lateral size greater than the lateral size of a wafer (or other object) to be polished. The large fluid cavity can provide a large area of uniform support pressure. In one embodiment of the invention, the large fluid cavity is surrounded by a support ring including fluid inlets connected to an independent fluid source. The support ring is outside the area of support for polishing pads in contact with a wafer, but fluid flow from the inlets in the support ring is connected to fluid source having a pressure independent of the pressure in the large fluid cavity. Thus, changing fluid pressure in the support ring can change the fluid film thickness (and support pressure) in the large cavity.
In accordance with yet another aspect of the invention, a hydrostatic bearing has a non-uniform support pressure profile but a wafer (or other object being polished) is moved so that average support pressure is constant across the wafer when averaged over the range of motion. One such hydrostatic bearing includes drain grooves that spiral from an outer region to a central region of the hydrostatic bearing. The spiral drain grooves may follow, for example, a path that is a part of a cardiod. Inlets arranged on concentric circles surrounding the central region have fluid pad areas with boundaries partially defined by the spiral drain grooves. These fluid pads extend along the spiral grooves so that the fluid pads associated with one ring of inlets extend to radii that overlap the radii of the fluid pads for adjacent rings of inlets. The fluid pads are further disposed so that the same percentage of each circumferential path about the center of the bearing is on or over fluid pads. Thus, each point on a wafer that is rotated about the center of the bearing experiences the same average pressure. This hydrostatic bearing can also be used with a support ring of independently controlled fluid inlets outside the outer region of the bearing.
In accordance with another aspect of the invention, a hydrostatic fluid bearing has constant fluid pressure at each fluid inlet and adjusts support pressure by changing the height of one or more inlets and fluid pads with respect to the object being supported. In various embodiments employing this aspect of the invention, a hydrostatic fluid bearing includes a set of inlet blocks where each inlet block includes one or more fluid inlet (and associated fluid pad). The inlet blocks are mounted on a mechanical system that permits adjustments of the relative heights of the inlet blocks. Such mechanical systems can be operated, for example, by air or hydraulic cylinders, piezoelectric transducers, or electrically power actuators or solenoids.
The various aspects of the invention can be employed alone or in combinations and will be better understood in view of the following description and accompanying drawings.